1. Field of the Invention
The present invention relates to a shift control system for an automatic transmission for an automobile which causes the automatic transmission to shift based on torque input to or output from the automatic transmission. More particularly, this invention relates to such an automatic transmission shift control system in which an automatic transmission shifts while producing less of a shift shock which is physically felt by passengers.
2. Description of Related Art
Automatic transmissions with torque converters for automotive vehicles typically perform shift operations in two consecutive phases. These phases include a torque shift phase and an inertia shift phase. Because little drop in turbine speed takes place in the torque shift phase, the automatic transmission is not subjected to shift shock in the torque shift phase. However, in the inertia phase, in which discharge of inertial energy takes place due to a change in gear ratio, the speed of the turbine goes down rapidly, and the automatic transmission is subjected to shift shock.
One technique to prevent an occurrence of shift shock during shifting is to monitor the output torque of the automatic transmission to lower a line pressure supplied to the automatic transmission at the moment the output torque converts to an increasing condition from a decreasing condition. Such a shift shock control technique is known from, for instance, Japanese Unexamined Patent Publication No. 62-41,458.
It has been found that an actual input torque supplied to an automatic transmission undergoes complicated changes. It is, therefore, difficult to eliminate or reduce shift shock by regulating a line pressure during a transition of the shift operation into an inertia phase which is detected based only on a change in torque.
FIG. 1 provides a brief background that will enhance an understanding of the operation of the shift control system in accordance with the present invention. More specifically, FIG. 1 is a time chart for various transmission control factors, such as a turbine speed N.sub.t of a torque converter, a line pressure (PL) duty control, a clutch pressure CL supplied to a clutch chamber, an output torque T.sub.P of an automatic transmission, and a turbine output torque T.sub.t of the torque converter. When the automatic transmission begins a shift at a time T.sub.A, a turbine output torque T.sub.t, transmitted to a transmission mechanism of the automatic transmission, is provided as an output torque T.sub.P of the automatic transmission and transmitted to drive wheels through a propeller shaft. Shift shock, physically felt by passengers, is produced by a steep decline in transmission output torque T.sub.P in a time period between times T.sub.B and T.sub.D during shifting. This time period changes due to wearing of frictional coupling elements of the automatic transmission and is different for various automatic transmissions. In order to effectively prevent shift shock from occurring, it is not sufficient to simply reduce the amplitude of such a steep decline in transmission output torque.